1. Field of the Invention
The present invention relates to diagnostic ultrasound system, or more particularly, to a diagnostic ultrasound system adaptable to tissue Doppler imaging based on an ultrasonic pulsed-wave Doppler technique.
2. Description of the Related Art
In the past, a diagnostic ultrasound system having a tissue Doppler imaging (TDI) feature has been disclosed, for example, in Japanese Patent Laid-Open No. 6-114059 (of which title of the invention is an xe2x80x9cultrasound color Doppler tomography systemxe2x80x9d) proposed by the present applicant. The diagnostic ultrasound system described in the unexamined patent publication has a feature that uses a pulsed-wave Doppler technique and a lowpass filter to detect the motion velocities of tissues including the cardiac muscle and vascular wall, compute various physical volumes relevant to motion on the basis of the motion velocities, and display the results of computation in appropriate modes in color. For detecting the motion velocity of a tissue, since the motion velocity of a tissue is markedly lower than a blood flow velocity, the pulse repetition frequencies (PRF) of transmitted ultrasonic pulsed waves (rate pulses) are lowered to enable measurement of super-low motion velocities of tissues.
Various modes are available for color display of the results of computation. In the invention described in the unexamined patent publication, two-dimensional color display has been proposed. As for the gradations for the color display, a procedure used for the blood flow imaging, which is implemented in a color Doppler system and shares concepts with tissue Doppler imaging, can be employed.
In the blood flow imaging, a band of Doppler shift frequencies fd ranging from xe2x88x92fr/2 to fr/2 (where, fr denotes a pulse repetition frequency of an ultrasonic pulsed wave) is rendered, as shown in FIG. 27, in 32 gradation (gray-scale)levels (fr/32 per level) with different color brightnesses or hues. In other words, a scale whose gradation levels associated with velocities (Doppler shifts) have a constantly progressive change is assigned to the whole band of Doppler shift frequencies ranging from xe2x88x92fr/2 to fr/2, thus defining a color-display gradation between red (yellow) to blue (light blue).
As mentioned above, one of the characteristics of tissue Doppler imaging lies in that pulse repetition frequencies (lower frame rates) of ultrasonic waves are set to lower values in order to enable measurement of ultra-low motion velocities of tissues. Owing to the characteristic of enabling measurement of an ultra-low motion velocity, the band of Doppler shift frequencies required for display images produced by tissue Doppler imaging is narrower than that required for display images produced by blood flow imaging of ranges, for example, from xe2x88x92fr/8 to fr/8.
Nevertheless, at present, the assignment of a color-display gradation adopted for blood flow imaging cannot help applying to tissue Doppler imaging as it is. As a result, the number of gradation levels assigned to a low-velocity band is quite limited. A tissue region to be observed; such as, the cardiac muscle appears, for example, in red of almost the same hue or brightness. It is therefore very hard to visually assess a difference in velocity in a low-velocity band image. Even if a difference in velocity smaller than fr/32 can be detected, since a displayed hue or brightness is unchanged, high-precision detectability is canceled out by poor displaying ability. There still exists an unsolved problem that the high-precision detectability cannot be exerted fully.
When tissue Doppler imaging is used for diagnosis, it should be discerned promptly whether the cardiac muscle or any other tissue region of interest is normal or abnormal. Using the conventional display technique, the levels of a color-display gradation are assigned uniformly between a low-velocity band and a high-velocity band. The displaying ability for the low-velocity band is, as described previously, poor. There is therefore difficulty in discerning whether a diagnostic region is normal or abnormal. Consequently, it takes too much time for diagnosis. Moreover, an examining physician is requested to have high expertise.
The present invention attempts to solve the aforesaid unsolved problems. The first object of the present invention is to improve the ability to display a low-velocity band image by making the most of the function relevant to measurements of motion concerning a low-velocity band which is available in tissue Doppler imaging.
The second object of the present invention is to achieve the first object and provide images that are produced by tissue Doppler imaging (hereinafter, TDI images) and facilitate easy discernment of whether a region of interest (hereinafter, ROI)is normal or abnormal.
For achieving the above objects, as one aspect of the invention, there is provided a diagnostic ultrasound system for displaying a color image of a motion of a tissue contained on a subject""s tomographic plane, comprising: an element for scanning an ultrasonic pulse signal along the tomographic plane so as to acquire an electrical echo signal corresponding to an ultrasonic signal reflected from the tomographic plane; an element for extracting a Doppler signal from the echo signal, the Doppler signal being Doppler-shifted by the motion of the tissue; an element for calculating velocity data concerning the motion of the tissue for respective sample points on the tomographic plane on the basis of the Doppler signal; an element for setting a scale along which each of the velocity data over a measurable band of frequencies of the Doppler signal is assigned to each gradation data for color display, the measureable band of frequencies being limited by a pulse repetition frequency of the ultrasonic pulse signal and a given low-velocity band of the measurable frequency band being enhanced in the gradation data than a remaining velocity band of the measurable band; an element for converting the velocity data into the gradation data according to the scale; and an element for displaying the color image using the gradation data provided by the velocity converting element.
Preferably, the extracting element comprises a low-pass filter for selectively extracting the Doppler signal. Still preferably, the scale is non-linear in a ratio between changes in the velocity data and changes in the gradation data. For example, the ratio in the given low-velocity band is higher than said ratio in the remaining velocity band. For example, the specified low-velocity band is any of xe2x88x92fr/8xe2x89xa6fdxe2x89xa6fr/8, xe2x88x92fr/12xe2x89xa6fdxe2x89xa6fr/12, and xe2x88x92fr/16xe2x89xa6fdxe2x89xa6fr/16, where fr represents the pulse repetition frequency of the ultrasonic pulse signal and fd represents a Doppler shift frequency. It is preferred that at least maximum data of the color code data is discontinuous in gradation levels from a series of remaining data of the color code data.
A diagnostic ultrasound system in accordance with the above aspect of the present invention is adaptable to tissue Doppler imaging. For this imaging technique, the cardiac muscle or any other region is scanned according to an ultrasonic pulsed-wave Doppler method. Echoes are then obtained, whereby a motion velocity is computed for each of sample points on a scanned tomographic plane. The motion velocity is visualized in two-dimensional color display mode. For the display, the slope of a scale of velocity data versus color-display gradation data to be assigned to a low-velocity band (for example,xe2x88x92fr/8xe2x89xa6fdxe2x89xa6fr/8) within a velocity range measurable by the ultrasonic pulsed-wave Doppler that is equivalent to a band of Doppler shifts; xe2x88x92fr/2xe2x89xa6fdxe2x89xa6 fr/2 (where fr denotes a pulse repetition frequency of an ultrasonic pulsed wave, and fd denotes a Doppler shift) is larger than that of the other velocity band. This results in an increase in display resolution for the low-velocity band. A minute change in low-velocity motion of the cardiac muscle is therefore visualized with high sensitivity as a change in multi-level gradation of color brightness degrees (luminances) or hues. Consequently, even if an attempt is made to upgrade the function relevant to measurements of low-velocity motion by specifying lower pulse repetition frequencies, the function will not be impaired. Moreover, a pixel rendering a velocity comparable to a maximum gradation level is displayed with such a hue as making the pixel discontinuous with the other pixels rendering lower velocities. The pixel rendering the velocity comparable to a maximum gradation level is therefore readily discernible. Thus, discernible efficiency improves.
As another aspect of the invention is provided by a diagnostic ultrasound system for displaying a color image of a motion of a tissue contained on a subject""s tomographic plane, the color image being superposed on a B-mode tomographic image of the subject""s tomographic plane, the system comprising: an element for scanning an ultrasonic pulse signal along the tomographic plane to acquire an electrical echo signal corresponding to a reflected ultrasonic signal from the tomographic plane; an element for extracting a Doppler signal from the echo signal, the Doppler signal being Doppler-shifted by the motion of the tissue; an element for calculating velocity data concerning the motion of the tissue for respective sample points on the tomographic plane on the basis of the Doppler signal; an element for forming data of the B-mode tomographic image on the basis of the echo signal; an element for blanking the velocity data at every sample point when each of the velocity data exceeds a specified threshold; and an element for displaying the color image by coloring the velocity data and by superimposing the velocity data subjected to blanking by the blanking element on the data of the B-mode tomographic image.
Still another aspect of the invention is provided by a diagnostic ultrasound system for displaying a color image of a motion of a tissue contained on a subject""s tomographic plane, the color being superposed on a B-mode tomographic image of the subject""s tomographic plane, the system comprising: an element for scanning an ultrasonic pulse signal along the tomographic plane to acquire an electrical echo signal corresponding to a reflected ultrasonic signal from the tomographic plane; an element for extracting a Doppler signal from the echo signal, the Doppler signal being Doppler-shifted by the motion of the tissue; an element for calculating velocity data concerning the motion of the tissue for respective sample points on the tomographic plane on the basis of the Doppler signal; an element for forming data of the B-mode tomographic image on the basis of the echo signal; an element for setting a scale along which each of the velocity data over a measurable band of frequencies of the Doppler signal is assigned to each gradation data for color display, said measurable band of frequencies being limited by a pulse repetition frequency of the ultrasonic pulse signal and a given low-velocity band of the measurable frequency band being enhanced in the gradation data than a remaining velocity band of the measurable band; an element for converting the velocity data into the gradation data according to the scale; an element for blanking either one of the converted gradation data and the calculated velocity data at every sample point when each of either one exceeds a specified threshold; and an element for displaying the color image by coloring the velocity data and by superimposing the velocity data subjected to blanking by the blanking element on the data of the B-mode tomographic image.
Preferably, the scale setting element is an element that sets the scale in which a ratio of changes in the gradation data to changes in the Doppler frequency is higher than a corresponding ratio for analysis of fluid motion within the subject and the velocity data larger than a reference velocity data corresponding to maximums of the gradation data are all assigned to the maximums. It is preferred that the diagnostic ultrasound system further comprises an element for setting the threshold independently of the scale. It is also preferred that the scale setting element is an element that automatically sets the threshold in connection with setting the scale.
In consequence, in the same way as explained above, an increase in display resolution for the low-velocity band is provided. In addition, sample points having velocities higher than the specified threshold on the tomographic plane dose not display the tissue Doppler image and, instead of it, display only the B-mode tomographic image as the background image hidden behind the tissue Doppler image. Properly specifying the threshold enables to exclude or minimize the meaningless (gradation-less) velocity color region of a tissue.